World War I Militaria
While World War II militaria is both more popular and more recent, thus more widely available, you may have a more valuable collection if you can get your hands on military memorabilia from World War I. Pieces from World War I typically command a much higher price in the open market. Make sure that the pieces you buy are in mint or near-mint condition, or else have a great story attached to them that explains the bullet holes.
Here are some good resources for adding to your World War I militaria collection, broken down by nationality.
America
This war, which President Woodrow Wilson entered only reluctantly, showcased the military strength of a young nation on the world stage. You can showcase your collection of the items they carried by visiting sites such as War Dog Militaria, which has plenty of vintage American military antiques in its kennel. These include a box that once held machine gun cartridges, as well as a World War I era grenade vest that an Army recruit would have worn.
Germany
Decades before Hitler took power, the Kaiser was rattling sabers all across Europe. You can purchase some of these sabers, along with bayonets and other weapons, at Great War Militaria. The catalogue here is quite impressive, with hundreds of available items. You can also visit Orth Militaria to obtain items such as the belt of a Prussian officer and the buckle of an enlisted man.
Great Britain
If you are into collecting the uniforms and weapons of the heroic RAF (Royal Air Force), as well as the British army and navy, look no further than Great War Militaria, which has an extensive assortment of British World War I militaria for sale. Items include a brown and green camouflage helmet and the cleaning kit from an Enfield rifle. Any serviceperson stationed in Afghanistan or Iraq receives free shipping on their militaria purchases.
France
The toll that World War I took on the French people, who held the front line against the German and Austrian side, was tremendous. You can see their sacrifice as well as their style in items such as a trench lighter with a dreaming soldier on one side and a weeping soldier on the other, available on Ruby Lane. Of particular interest is a George Benjamin Clemenceau pin that would have been worn by a French serviceman in the field during the final year of the war.